Phobos: A Chip Off of Mars

NASA's Mars Reconnaissance Orbiter captured this view of the Martian moon Phobos on March 23, 2008, from a distance of about 4,200 miles. It's actually a false-color view, combining data from the camera's blue-green, red, and near-infrared channels.The smallest resolved features are about 65 feet across

Asaph Hall's discovery of Mars's two small moons in 1878 is one of the greatest success stories of observational astronomy. For nearly a century thereafter, however, Phobos and Deimos were little more than dynamical curiosities that traveled oh-so-close to their parent planet.

With the Space Age came the opportunity to see these objects at closer range, and a succession of spacecraft (beginning with Mariner 9 in 1971) has revealed Deimos and especially Phobos with ever-greater detail. Some particularly dramatic views have come from the HiRISE camera aboard Mars Reconnaissance Orbiter.

Yet, despite decades of careful scrutiny from space and from ground-based telescopes, astronomers still debate where these little worlds came from. They're very dark, implying a carbon-rich composition akin to bodies at the outer margins of the asteroid belt. Both also have a low density, suggesting they're riddled with internal cavities. All this evidence points to the notion that Mars somehow captured a couple of wandering objects that happened to stray too close by. (One big "gotcha": both moons have extremely circular orbits very close to Mars's equatorial plane — not exactly the outcome you'd get from two random encounters.)

Now a team of observers has stirred up the pot of possibilities. Yesterday, at the European Planetary Science Congress in Rome, hometown researcher Marco Giuranna (IFSI/INAF) argued that far-infrared spectra of Phobos acquired with the Mars Express orbiter don't match the composition of any known chondritic asteroid or meteorite type. (Chondrites are rocky bodies that have remained unaltered since the solar system's formation.)

As seen by a spectrometer aboard the Mars Express orbiter, the thermal-infrared spectrum of Phobos is a close match to certain phyllosilicates (clay minerals). Also, the position of the peak labeled CF suggests a composition rich in ultramafic (iron- and magnesium-rich) minerals.

What he and others have found instead is a distinct composition rich in dark ultramafic minerals (having lots of iron and magnesium) and clay minerals, called phyllosilicates. The clay signature appears strongest near the large crater Stickney, hinting that deposits were dredged up from deeper down.

Moreover, Martin Pätzold (Köln University) also announced at the ESPC that Phobos has a density of just 1.86 ±0.02 g/cm3. To be so low, the moon's interior must be incredibly porous, with voids taking up 25% to 35% of the total volume. The new density value is based on careful tracking of Mars Express during close flybys of Phobos, including a brush-by just 42 miles (67 km) away last March 3rd.

That mix of clay and ultramafic minerals might be rare among the asteroids, but it's likely a lot more common on the Martian surface down below. The upshot, Giuranna suggests, is that Phobos wasn't captured but more likely formed in place from debris blasted off the surface of Mars during a large, long-ago collision. Most of the debris would have escaped to interplanetary space, but enough of it (11 trillion tons, more or less) hung around to recollect into the two Martian moons.

It's a dramatic hypothesis, to be sure, but is it the correct one? First, as Giuranna's team points out, the new compositional clues don't rule out that Phobos (and Deimos) were captured. Observers have found plenty of asteroids with clay minerals on their surfaces, and ultramafic meteorites (achondrites) aren't exactly rare. Second, the moons' spongy interiors could have resulted if a single body broke apart during its capture by Mars and then then reassembled into a pair of not-quite-solid satellites. Finally, someone needs to run a computer simulation to see if this smash-and-dash scenario makes dynamical sense (according to impact modeler Robin Canup, no one's done it yet).

In any case, we might not have to wait too long to learn the Martian moons' pedigree. A Russian mission called Phobos-Grunt is being readied for launch late next year. The effort has encountered technical problems — it was supposed to head off toward Mars a year ago — but if it succeeds, scientists will have 100 to 200 grams of Phobos to analyze when the sample-return capsule lands in July 2014.

Jupiter Shines Extra Bright

If you look up on any clear September night, a big bright “star” will greet you. It’s low in the east after twilight, and higher in the southeast as the evening grows late. This is the planet Jupiter, and it's far brighter than any true star in the night sky.

Jupiter is always bright, but if you think it looks a little brighter than usual this month, you’re right. Jupiter is making its closest pass by Earth for the year. And this year’s pass is a little closer than any other between 1963 and 2022.

Jupiter is nearest to Earth on the night of Monday, September 20th: 368 million miles away. But it remains nearly this close and bright (magnitude -2.9) throughout the second half of September.

At the closest point of its previous swing-by, in August 2009, Jupiter was about 2% farther from both Earth and Sun than this time. That made it 8% dimmer. At its next pass, in October 2011, it will be 0.4% more distant than now.



Jupiter on Sepetember 2, 2010
Jupiter on September 2nd, shown with south up, as it appears in a reflecting telescope. The Great Red Spot is prominent at upper left, and the South Equatorial Belt is almost invisible.

Also, according to legendary planetary observer Richard Schmude, Jupiter is an additional 4% or so brighter than usual because one of its brown cloud belts has gone missing. For nearly a year the giant planet's South Equatorial Belt, usually plain to see in a small telescope, has been hidden under a layer of bright white ammonia clouds.

Because Jupiter is so close to Earth, this is a great opportunity to view it through a telescope. Jupiter is most interesting when the Gred Red Spot is visible and/or when one of the moons is casting a shadow on Jupiter's disk.

Encounters with Comet Hartley 2

An icy visitor is positioning itself for easier viewing in the coming weeks. Periodic Comet 103P/Hartley 2 won't have the pizzazz of Comet Hale-Bopp or the unexpected spectacle of Comet Holmes. But it will be high in the evening sky when at its best, glowing at perhaps 5th magnitude. It should be dimly visible to the unaided eye from very dark locations, and visible in binoculars and telescopes from almost anywhere in the Northern Hemisphere. (Most of you in the Southern Hemisphere will be able to observe it from mid-October onward.)

Hartley 2's brightness, and its unusually fast slide across the constellations, both result from how closely it will approach Earth: by just 0.12 astronomical unit (11 million miles; 18 million km) on October 20th. This will be its closest approach since its 1986 discovery and one of the closest approaches of any comet in the last few centuries.

The comet has reached 9th magnitude and is brightening by 0.1 magnitude per day. So right now, before the Moon washes the sky with light, is an especially good time to look for this faint visitor.

By September 1st Hartley 2 had climbed north into a corner of Lacerta, where it spent a few days before crossing into northern Andromeda. On the night of September 8-9, at new Moon, the comet was less than 1° from 3.6-magnitude Omicron (ο) Andromedae.

The waxing Moon will brighten the evening sky from about September 15th to 26th. On the 22nd Hartley 2 should be 7th or 8th magnitude and within a few degrees of Lambda (λ) Andromedae.

October 1st finds the comet passing 1.5° south of 2.2-magnitude Alpha (α) Cassiopeiae, high in the northeast during moonless evenings. Perhaps 6th magnitude by then, it should remain at least this bright for the next nine weeks. But it's important to note that, with the comet now just 0.18 a.u. from Earth and closing, its light is no longer concentrated into a small dot but instead is more spread out. So even if you can sight a 6th-magnitude star with the unaided eye, Hartley 2 will be tougher. It's closest to Earth on October 20th at a distance of just 0.121 a.u.

On the night of October 7th in the Americas, when the comet should be 5th or 6th magnitude, it creeps less than 1° south of the Double Cluster in Perseus, magnitudes 4.3 and 4.4. This will make for a wonderful wide-field sight and a great astrophoto opportunity — particularly since it's again new Moon!

From here on Hartley 2 turns southeast, passing near the head of Perseus. On October 20th the fuzzy visitor passes just south of brilliant Capella. By the end of October the comet should still be around 5th magnitude — but now in Gemini. So it doesn't gain a high altitude until later in the night. Perihelion, 1.06 a.u. from the Sun, comes on the 28th — but that morning the nearly last-quarter Moon is just a few degrees away.

Moonless viewing times return around November 1st. But now, with the comet moving away from both the Sun and Earth, it fades by about a magnitude every two weeks. Besides — by then our attention should surely be turning to the exploits of (and pictures from) NASA's EPOXI spacecraft, which swoops by it on November 4th at a distance of just 600 miles (1,000 km).

A Comet's Tale

How could a short-period comet, visible to the unaided eye, go undiscovered until just 24 years ago? Read on.

Malcolm Hartley first spotted it on March 16, 1986, at magnitude 17 or 18 during a sky survey by the 1.2-meter U.K. Schmidt telescope at Siding Spring, Australia. A series of position measurements soon revealed it to be a short-period comet orbiting the Sun about every 6 years. It was the second short-period comet discovered solely by Hartley, hence the "2" in its name. The appellation 103P indicates that it was the 103rd comet with a known orbital period.

A backtrack of Hartley 2's path revealed that it fell into its current orbit only recently. Three close encounters with Jupiter (0.33 a.u. in 1982, 0.09 a.u. in 1971, and 0.23 a.u. in 1947) had shifted the comet's track closer to the Sun. Prior to those encounters, Hartley 2 never came closer than 2 a.u. from the Sun, leaving it beyond visual detection.

Hartley 2's next return came in 1991, when it brightened that September to 8th magnitude. It did so again at its following return in December 1997. The 2004 apparition was a poor one, with the comet far from Earth.

Now it's arriving front and center for its best showing yet. So enjoy it while you can!

Hubble Revisits Supernova 1987A

Supernova 1987A's ring, about a light-year across,
was probably shed by the star about 20,000 years before it exploded.
The dozens of bright spots around the ring mark where a shock wave
unleashed by the stellar blast is slamming into the ring's material.

It's been more than two decades since we Earthlings got word that a star in the Large Magellanic Cloud had blown itself to smithereens.

Supernova 1987A peaked at 3rd magnitude, making it a snap to spot by eye. But, with its declination of -69°, the blast was invisible to virtually everyone north of the equator. I'm jealous of my southern astro-friends because I never got to see it. (In fact, I wonder how the popular perception of and appreciation for astronomy might be different had this event been in view from northern skies — a topic for another day!)

Fortunately, the Hubble Space Telescope started observing SN 1987A within months of its launch in 1990. Those first views revealed a ring of matter thrown out by the star about 20,000 years before its demise. The supernova's expanding shock wave eventually crashed into that ring, creating a necklace of bright knots first spotted in 1995 HST images. Observers have kept tabs on this ring ever since — primarily with an instrument called the Space Telescope Imaging Spectrograph, or STIS, which astronauts installed on Hubble in 1997.

Unfortunately, an electronic failure in 2004 rendered STIS inoperable, and astronomers had to make do without its services until last year, when spacewalking astronauts replaced a faulty circuit board and brought STIS back to life.

In the September 2nd edition of Science Express, a star-studded international team of astronomers describe observations of the supernova made with STIS earlier this year, the first in six years. The ejected ring is still there, now studded with about 30 hotspots. Over time, as the supernova's shock wave continues to barrel outward, these should merge into a single bright band.

More interesting is the insight being gleaned from a second shock wave, this one triggered by the ring itself and propagating back toward what's left of the progenitor star and through the supernova's expanding debris. The team reports that spectra of this inner shock reveal lots of hydrogen, as you'd expect, but they also see some other emissions that are probably from nitrogen and perhaps from carbon.

Essentially, the now-gone star has laid bare whatever was inside when it exploded, and over time careful observations by HST and other telescopes will, in Humpty Dumpty fashion, attempt to put the progenitor star back together again.

"I think a great thing here is the resurrection of STIS," notes coauthor Robert Kirshner (Harvard-Smithsonian Center for Astrophysics). "Astronauts zipping out 114 screws while wearing boxing gloves were not just doing it for the challenge! This paper shows that the instrument is back working, and that we're finding out new things about an object that is about the same age as HST."